Some advanced vehicle control systems include vehicle stability control logic that may assist in keeping a vehicle within a defined performance region. For example, some such systems may calculate a corrective yaw moment of the vehicle and apply brakes to the appropriate wheels when the vehicle exceeds a threshold condition. An example of such a system is described in U.S. Pat. No. 6,547,343 entitled “Brake System Control”, of common assignment herewith. Other systems have been contemplated that attempt to augment driver steering commands using a special-purpose assisted steering control system.
In an advanced steering control system, such as Active Front Steering (AFS), a change angle (delta angle) may be added or overlaid to a road wheel angle of a vehicle to produce an augmented total road wheel angle. The driver of the vehicle typically uses a hand wheel or steering wheel to attempt to control the road wheel angle of the vehicle. The delta angle may be calculated in response to various driving conditions, such as a rapid turn or an emergency maneuver, and overlaid to improve vehicle stability and performance. The technique of adding a delta angle to the road wheel angle is also known as angle overlay. Although angle overlay may improve vehicle stability and performance, it also results in a variable relationship between the hand wheel angle and the road wheel angle. Furthermore, such steering control systems may require the use of special purpose hardware, resulting in additional cost and overall vehicle weight.
In some driving conditions, there is a need to impart varying lateral force or torque to the vehicle without changing the relationship between the hand wheel angle and the road wheel angle. Exemplary scenarios include maintaining a relative position within a driving lane, parking assistance, wind gust mitigation, and other stability control scenarios. An angle overlay system may impart additional torque on road wheels, but the primary purpose of the system is to alter the road wheel angle. To control a steering system based upon changes in torque, a new method must be developed independent of an angle overlay system. Additional advantages may be realized through imparting additional positive or negative torque using existing steering control systems, such as electric power steering (EPS) or hydraulic power steering (HPS) systems.
Accordingly, there is a need for a method for calculating a torque overlay command in a steering control system.